Different functional layers in different organic semiconductor devices request a variety of special characteristics.
For instance OTFTs need high mobility materials in their active channel. Transparent circuits, such as transparent OTFTs require that the high mobility organic material also comprises a wide electronic band gap; the electric injection of holes and/or electrons must be still provided.
Solar cells and OLEDs require transparent transport layers, with high conductivity. The transparency is necessary in those opto-electric devices to avoid non desired absorption of the light. These so called “window” materials can be used as transport layers, exciton or charge blocking layers. The thickness of the layers made with the window materials is used to adjust the micro cavity of the OLEDs and solar cells in such a way that the outcoupled emission of the OLED is a maximum, and in the case of the solar cell, the absorption and respective photocurrent of the device is a maximum. The non-optically active layers of all kinds of semiconductor devices can be exchanged for window materials in order to fabricate fully transparent components and circuits (e.g US20060033115).
The functionality and nomenclature of the layers are typical as used in the field. Further explanation can be found in US2006244370.
Electronic devices also need high stability towards temperature, meaning that the intrinsic properties of the amorphous organic semiconducting materials, such as triphenyl amine derivatives, or phenantronine derivatives, must include a high glass transition temperature (Tg) and high temperature stability in the device.
The conductivity can be, for example, measured by the so-called 2-point or 4-point-method. Here, contacts of a conductive material, such as gold or indium-tin-oxide, are disposed on a substrate. Then, the thin film to be examined is applied onto the substrate, so that the contacts are covered by the thin film. After applying a voltage to the contacts the current is measured. From the geometry of the contacts and the thickness of the sample the resistance and therefore the conductivity of the thin film material can be determined. The four point or two point method give the same conductivity values for doped layers since the doped layers grant a good ohmic contact.
The temperature stability can be also measured with that method in that the (undoped or doped) layer is heated stepwise, and after a waiting period the conductivity is measured. The maximum temperature, which can be applied to the layer without loosing the desired semiconducting properties, is then the temperature just before the conductivity breaks down. For example, a doped layer can be heated on the substrate with two electrodes, as disclosed above, in steps of 1° C., wherein after each step there is a waiting period of 10 seconds. Then the conductivity is measured. The conductivity changes with temperature and breaks down abruptly at a particular temperature. The temperature stability is therefore the temperature up to which the conductivity does not break down abruptly. The measurement is performed in vacuum.
The properties of the many different used materials can be described by the position of their highest occupied molecular orbital energy level (HOMO, synonym of ionization potential), and the lowest unoccupied molecular orbital energy level (LUMO, synonym of electro affinity).
A method to determine the ionization potentials (IP) is the ultraviolet photo spectroscopy (UPS). It is usual to measure the ionization potential for solid state materials; however, it is also possible to measure the IP in the gas phase. Both values are differentiated by their solid state effects, which are, for example the polarization energy of the holes that are created during the photo ionization process (N. Sato et al., J. Chem. Soc. Faraday Trans. 2, 77, 1621 (1981)). A typical value for the polarization energy is approximately 1 eV (E. V. Tsiper et al., Phys. Rev. B 195124 (2001)), but larger discrepancies of the values can also occur (N. Sato et al., J. Chem. Soc. Faraday Trans 2, 77, 1621 (1981)). The IP is related to beginning of the photoemission spectra in the region of the large kinetic energy of the photoelectrons, i.e. the energy of the most weakly bounded electrons. A related method to UPS, the inverted photo electron spectroscopy (IPES) can be used to determine the electron affinity (EA) (see e.g. W. Gao et. al, Appl. Phys. Lett. 82, 4815 (2003). However, this method is less common. Electrochemical measurements in solution are an alternative to the determination of solid state oxidation (Eox) and reduction (Ered) potential. An adequate method is for example the cyclovoltammetry (e.g. J. D. Anderson, J. Amer. Chem. Soc. 120, 9646 (1998)). Empiric methods for the extraction of the solid state ionization potentials are known from the literature. (e.g. B. W. Andrade et al., Org. Electron. 6, 11 (2005); T. B. Tang, J. Appl. Phys. 59, 5 (1986); V. D. Parker, J. Amer. Chem. Soc. 96, 5656 (1974); L. L. Miller, J. Org. Chem. 37, 916 (1972), Y. Fu et al., J. Amer. Chem. Soc. 127, 7227 (2005)). There are no known empiric equations for the conversion of reduction potentials into electro affinities. The reason for that is difficulty of the determination of the electro affinity. Therefore, a simple rule is used very often: IP=4.8 eV+e*Eox (vs. Ferrocen/Ferrocenium) and EA=4.8 eV+e*Ered (vs. Ferrocen/Ferrocenium) respectively (see B. W. Andrade, Org. Electron. 6, 11 (2005) and Refs. 25-28 therein). Processes are known for the correction of the electrochemical potentials in the case other reference electrodes or other redox pair are used (see A. J. Bard, L. R. Faulkner, “Electrochemical Methods: Fundamentals and Applications”, Wiley, 2. Ausgabe 2000). The information about the influence of the solution used can be found in N. G. Connelly et al., Chem. Rev. 96, 877 (1996). It is usual, even if not exactly correct to use the terms “Energy of the HOMO”E(HOMO) and “energy of the LUMO” E(LUMO) respectively as synonyms for the ionization energy and electro affinity (Koopmans Theorem). It has to be taken in consideration, that the ionization potentials and the electron affinities are given in such a way that a larger value represents a stronger binding of a released or respectively of an absorbed electron. The energy scale of the molecular orbitals (HOMO, LUMO) is opposed to this. Therefore, in a rough approximation, is valid: IP=−E(HOMO) und EA=−E(LUMO). The given potentials correspond to the solid-state potentials. Hole transport layers, including the respective blockers, mostly have HOMO in the range from 4.5 to 5.5 eV (below the vacuum level) and LUMO in the range of 1.5 eV to 3 eV. The HOMO levels of the emitter materials are in the range of 5 eV to 6.5 eV, and the LUMO in the range from 2 to 3 eV. Electron transport materials, including their respective blockers, have their HOMO in range of 5.5 eV to 6.8 eV and LUMO in the range of 2.3 eV to 3.3 eV, larger (lower laying) LUMO and HOMO levels may be required for solar cells. The work function of the contact materials is around 4 to 5 eV for the anode and 3 to 4.5 eV for the cathode.
The performance characteristics of (opto)electronic multilayered components are determined from the ability of the layers to transport the charge carriers, amongst others. In the case of light-emitting diodes, the ohmic losses in the charge transport layers during operation are associated with their conductivity. The conductivity directly influences the operating voltage required and also determines the thermal load of the component. Furthermore, depending on the charge carrier concentration in the organic layers, bending of the band in the vicinity of a metal contact results which simplifies the injection of charge carriers and can therefore reduce the contact resistance. Similar deliberations in terms of organic solar cells also lead to the conclusion that their efficiency is also determined by the transport and extraction to the electrode properties of charge carriers.
By electrically doping hole transport layers with a suitable acceptor material (p-doping) or electron transport layers with a donor material (n-doping), respectively, the density of charge carriers in organic solids (and therefore the conductivity) can be increased substantially. Additionally, analogous to the experience with inorganic semiconductors, applications can be anticipated which are precisely based on the use of p- and n-doped layers in a component and otherwise would be not conceivable. The use of doped charge-carrier transport layers (p-doping of the hole transport layer by admixture of acceptor-like molecules, n-doping of the electron transport layer by admixture of donor-like molecules) in organic light-emitting diodes is described in US2008203406 and U.S. Pat. No. 5,093,698.
US2008227979 discloses in detail the doping of organic transport materials, also called matrix, with inorganic and with organic dopants. Basically, an effective electronic transfer occurs from the dopant to the matrix increasing the Fermi level of the matrix. For an efficient transfer in a p-doping case, the LUMO energy level of the dopant must be lower laying or at least slightly higher, not more than 0.5 eV, to the HOMO energy level of the matrix. For the n-doping case, the HOMO energy level of the dopant must be higher laying or at least slightly lower, not lower than 0.5 eV, to the LUMO energy level of the matrix. It is furthermore desired that the energy level difference for energy transfer from dopant to matrix is smaller than +0.3 eV.
The dopant donor is a molecule or a neutral radical or combination thereof with a HOMO energy level (ionization potential in solid state) lower than 3.3 eV, preferably lower than 2.8 eV, more preferably lower than 2.6 eV and its respectively gas phase ionization potential is lower than 4.3 eV, preferably lower than 3.8 eV, more preferably lower than 3.6 eV. The HOMO of the donor can be estimated by ciclo-voltametric measurements. An alternative way to measure the reduction potencial is to measure the cation of the donor salt. The donor has to exhibit an oxidation potential that is smaller or equal than −1.5 V vs Fc/Fc+(Ferrum/Ferrocenium redox-pair), preferably smaller than −1.5 V, more preferably smaller or equal than approximately −2.0 V, even more preferably smaller or equal than −2.2 V. The molar mass of the donor is in a range between 100 and 2000 g/mol, preferably in a range from 200 and 1000 g/mol. The molar doping concentration is in the range of 1:0000 (dopant molecule:matrix molecule) and 1:2, preferably between 1:00 and 1:5, more preferably between 1:100 and 1:10. In individual cases doping concentrations larger than 1:2 are applied, e.g. if large conductivities are required. The donor can be created by a precursor during the layer forming (deposition) process or during a subsequent process of layer formation (see DE 10307125.3). The above given value of the HOMO level of the donor refers to the resulting molecule or molecule radical.
A dopant acceptor is a molecule or a neutral radical or combination thereof with a LUMO level larger than 4.5 eV, preferably larger than 4.8 eV, more preferably larger than 5.04 eV. The LUMO of the acceptor can be estimated by cyclo-voltammetric measurements. The acceptor has to exhibit a reduction potential that is larger or equal than approximately −0.3 V vs Fc/Fc+(Ferrum/Ferrocenium redox-pair), preferably larger or equal than 0.0 V, preferably larger or equal than 0.24 V. The molar mass of the aceptor is preferably in the range of 100 to 2000 g/mol, more preferably between 200 and 1000 g/mol, and even more preferably between 300 g/mol and 2000 g/mol. The molar doping concentration is in the range of 1:0000 (dopant molecule:matrix molecule) and 1:2, preferably between 1:00 and 1:5, more preferably between 1:100 and 1:10. In individual cases doping concentrations larger than 1:2 are applied, e.g. if large conductivities are required. The acceptor can be created by a precursor during the layer forming (deposition) process or during a subsequent process of layer formation. The above given value of the LUMO level of the acceptor refers to the resulted molecule or molecule radical.
Typical examples of doped hole transport materials are: copperphthalocyanine (CuPc), which HOMO level is approximately 5.2 eV, doped with tetrafluoro-tetracyanoquinonedimethane (F4TCNQ), which LUMO level is about 5.2 eV; zincphthalocyanine (ZnPc) (HOMO=5.2 eV) doped with F4TCNQ; pentacene, with its HOMO around 4.6 eV, doped with tris {2,5-bis(3,5-bis-trifluoromethyl-phenyl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaazatriphenylene, which has its LUMO level at about 4.6 eV; a-NPD doped with 2,2′-(perfluoronaphthalene-2,6-diylidene) dimalononitrile.
Typical examples of doped electron transport materials are: fullerene C60 doped with acridine orange base (AOB); perylene-3,4,9,10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA) doped with leuco crystal violet; 2,9-di (phenanthren-9-yl)-4,7-diphenyl-1,10-phenanthroline doped with Tetrakis (1,3,4,6,7,8-Hexahydro-2H-pyrimido[1,2-a]pyrimidinato) ditungsten (II) (W(hpp)4); naphthalene tetracarboxylic acid di-anhydride (NTCDA) doped with 3,6-bis-(dimethyl amino)-acridine; NTCDA doped with bis(ethylene-.dithio) tetrathiafulvalene (BEDT-TTF).
There is a technical challenge to provide electron transport materials (ETM) and emitter host (EMH) materials that have a sufficiently low laying LUMO level so that they can be doped, and still have a enough high laying LUMO level which can efficiently transfer charge to emitter host (in case of an ETM) and transfer energy to the emitter dopant (in case of EMH). The limitation for high laying LUMO level of the ETL is given by the dopability, since the ndopants with very high HOMO tend to be unstable; also the injection is difficult for very high LUMO of the ETL.
A technical challenge for organic solar cells is to provide electron transport materials with low laying LUMO level that can easily align with the LUMO of the heterojunction acceptor of the solar cell. Furthermore, materials are required with the low laying LUMO level and a very low laying HOMO level that can be used to block holes.